Luminaires, or lighting fixtures, available in the market today are generally either open fixtures or closed fixtures. As can be seen in FIG. 1, open fixtures 102 are those with the optical system open to the environment, whereas closed fixtures 104 are sealed. The optical system is generally comprised of a lamp 106 and a reflector 108. In closed fixtures 104, a glass, plastic or other translucent or transparent lens 110 encloses the reflector 108 to allow for light to exit the aperture 112.
Open luminaires 102 incorporate glass, plastic or metal reflective optics 108. In many instances, these designs may have inherent challenges that affect the fixture. For instance, in many situations plastic yellows or discolors from ultraviolet (UV) exposure and heat resulting in decreased reflective properties. Plastic may also exhibit a static charge build-up, especially when exposed to moving air. The static charge increases dirt particle buildup through ionic attraction on the plastic, further reducing light transmission and reflection and exacerbating discoloring because of increased heat buildup. In some installations, use of UV-resistant acrylic compounds may delay the discoloring effect, but the material still degrades over time. Optics 108 comprised of glass generally do not degrade and stay clean longer due to the non-static properties of glass.
In some instances the reflective optics 108 are comprised of metallic materials. While metallic reflective optics generally do not degrade from UV exposure, they may be vulnerable to oxidation, which attacks the coatings used to cause reflectivity. Also, ungrounded metal may exhibit a static charge such that dirt particles are attracted to the reflective surface, accumulate, and reduce optical performance. Cleaning or wiping away the dirt from a specular metal surface is laborious and may create scratches on the surface, further degrading reflective performance.
In other instances reflective optics comprise glass or plastic coated with specular metal (through processes such as sputtering or vapor deposition), thereby creating reflectivity. While this approach may overcome some of the challenges described above, it is expensive, is geometry-dependent and is highly susceptible to damage such as scratches, chemical breakdown and dirt depreciation.
Furthermore, it is generally recognized that the optical performance of all luminaires changes over time depending upon the environment in which they are placed. Luminaire dirt depreciation (“LDD”) is one of the many factors used by the lighting industry to determine how many luminaires are needed to generate the recommended amount of light for the situation. Generally, the higher the LDD, the better the luminaire performs over time, thereby reducing the required fixtures needed in an installation. Studies conducted by groups such as the Illuminating Engineering Society of North America (IESNA) show that luminaires have different rates of performance deterioration due to dirt accumulation depending upon the cleanliness of the environment and the configuration of the fixture.
Productivity decreases with dropping light levels and maintenance is required to clean away the dirt and increase performance. Plastic lenses must generally be replaced on a periodic basis (e.g., every 3-5 years), all which adds up to extra cost for the owner. Therefore, what is needed is an inexpensive, reflector-based luminaire that overcomes many of the challenges found in the art, some of which are described above.